We report a case of a man positive for the human immunodeficiency virus who rapidly died of lymphoma with cerebral, lung, and pericardic involvement. After autopsy, histopathological study of the different tumor sites showed the same morphological feature of immunoblastic lymphoma, with large areas of necrosis. In situ hybridization showed monotypic kappa light chain mRNA within lung lymphoma cells and monotypic lambda light chain mRNA within cerebral lymphoma cells. Polymerase chain reaction analysis showed two different immunoglobulin heavy chain gene rearrangements for lung and cerebral lymphoma. Here the simultaneous association of two malignant lymphomas derived from different B-cell clones indicates that molecular analysis of different tumor sites can distinguish between dissemination of the same lymphoma and simultaneous proliferation of different malignant B-cell clones.